It is known in the art to manufacture enals via catalytic reaction of aldehydes, aldehyde mixtures or aldehyde/alcohol mixtures. In a typical process, reactant aldehydes/alcohols are fed to a reactor. If a homogenous catalyst is used, it is co-fed to the reactor. If a heterogenous catalyst is used it is charged to the reaction vessel prior to start-up. The reaction products are then fed to a separation train wherein water, light, and heavy by-products are removed from desired products of the reaction. This train typically consists of at least two distillation columns for the removal of heavy and light boiling component. A decanter may be inserted between the reactor and distillation device to assist in removing any free water present in the reaction product. When using a homogenous catalyst, the aqueous phase contains catalyst and may be recycled to the reactor feed. A purge stream can be taken off this recycle. This general process and equipment configuration is diagrammed generically in FIG. 1 herein. The reaction is a condensation reaction and is often referred to the art as “aldol condensation”. Aldol condensation is used to describe a class of condensation reactions wherein a carbonyl reacts with another carbonyl or an alkenol to form a hydroxylaldehyde or hydroxyketone which is then (with sufficiently high reaction system temperature) subjected to dehydration to give an enal or enone product, respectively.
Specific examples and variations of the above generic approach are described for example in U.S. Pat. Nos. 4,684,750; 5,227,544; 8,791,304 B2 and 9,181,156 B2. More specifically, the '750, '544 and '156 patents describe the aldol condensation of n-butanal with itself in the presence of an alkali catalyst to form butyraldol and the subsequent dehydrogenation to form 2-ethylhexanol and the '304 patent describes the use of a continuous-stirred tank reactor to conduct the aldol condensation.
The prior art processes for forming enals from aldehydes using aldol condensation have a number of drawbacks. First, they typically require multiple complex engineered vessels and pieces of processing equipment assembled in series, which translates to a large initial capital expenditure for plant construction and significant ongoing equipment maintenance, repair and/or replacement costs. Further, each piece of equipment is typically designed and/or operated to facilitate long residence times and other conditions that can be required to maximize reactant conversion. For example, reactors are typically stirred tank reactors with very large volumes or pipe reactors with very long pipe runs and/or multiple catalyst injections. Similarly, distillation columns are often operated at high recycle levels such that the majority of the reactor effluent is returned to the reactor feed. The push to maximize conversion often results in extremely high energy use (and energy cost) as well as extended periods, sometimes hours, wherein reactants, products, and catalyst are all simultaneously exposed to each other. This extended exposure can be highly undesirable and result in unintended side-reactions, production of contaminating by-products and reduction of yield and production efficiency.
A continuing need therefore exists for a method for forming aldol condensation products that has lower capital construction, maintenance, energy use and operating costs while achieving high product conversion and yields with minimized contaminant and/or by-product formation.